1. Field of the Invention
The invention relates generally to the field of downhole hydrocarbon recovery and formation analysis and in particular to a method and apparatus for measuring acoustic mud velocity and acoustic caliper in the downhole environment.
2. Description of the Related Art
Borehole caliper is an important factor in the available accuracy and effectiveness of downhole data gathering instruments. Spatial irregularities in the borehole walls can adversely affect data integrity, unless these irregularities are detected and accounted for in data processing and/or acquisition. Borehole rugosity adversely affects downhole data measurements which are designed to assess the potential for hydrocarbon bearing formations adjacent a borehole. For example, cavities in the borehole wall can adversely affect measurements taken by downhole devices such as Nuclear Magnetic Resonance (NMR) devices. Thus, there is a need for an accurate downhole measurement of borehole rugosity.
NMR devices are typically used in determining properties for an adjacent formation, such as porosity of the material, permeability, the bound liquid volume, the clay bound volume (CBW) and bulk volume irreducible (BVI), as well as formation type and oil content. The principle of NMR works because atomic nuclei contain magnetic moments associated with their nuclear spin. When these nuclei are subjected to an applied static magnetic field, their magnetic moments tend to align either parallel or anti-parallel to this field. In a typical NMR device used in logging, a permanent magnet produces the static magnetic field and establishes the direction of orientation for the magnetic moments in a given volume of space. Typically in the art, a transmitter coil is placed near this region in order to induce a RF magnetic flux into this volume by means of the circuitry to which it is attached. The transmitter coil is oriented such that the magnetic field it induces into the volume lies substantially in the plane that is perpendicular to the static magnetic field. This volume of space where the two magnetic fields are essentially orthogonal is herein referred to as the sensitive region. By applying this RF magnetic field, the RF field can rotate the nuclear spin vectors within the sensitive region out of alignment with the static field.
Typically, the RF transmitter coil induces a RF magnetic pulse whose duration is timed to reorient the magnetic moments of the nuclei along a direction that is orthogonal to the static field of the permanent magnet. Once the spin moments are perpendicular to the static field and the RF pulse is removed, the magnetic moments undergo two notable processes. Firstly, the spins realign along the direction of the static magnetic field. This decay back along the direction of the static field occurs over a characteristic time scale called the spin-lattice relaxation rate, T1. Secondly, since the magnetic moments are non-aligned with the static field, they experience a perpendicular force which causes them to precess around the static field. This rate of precession is known as the Larmor frequency and is proportional to the strength of the static field. The decay of the spin magnetization in the plane perpendicular to the static field is known as the spin-spin decay and is characterized by its decay rate, T2.
Each molecule has its own characteristic values of T1 and T2. The practitioner of the art is skilled in interpreting NMR logging results in order to determine the composition of materials inside the sensitive region. If the sensitive region is comprised of many types of materials, the signal is an accumulation of all of their signals.
Typically, it is desirable to contain the entire sensitive region within the-rock formation. In most geometries, the sensitive region is a cylindrical shell which is coaxial to the permanent magnet, although other spatial arrangements can be produced. Since the sensitive region lies close to the surface of the borehole cavity, geometric anomalies in the surface of the wellbore can cause portions of the sensitive region to lie inside the borehole cavity rather than inside the rock formation, causing NMR signals to be received from what is contained inside the borehole, usually drilling mud. Drilling muds are oil or water based and hence have a large number of hydrogen nuclei: these are a strong source of contaminating NMR spin echo signals that may be stronger than the desired signals from the rock formation. Prior methods have used caliper arms to determine the distance from the tool body to the borehole wall, however, such methods are crude and do not yield precise measurements. Thus, there is a need for a method and apparatus to precisely determine borehole caliper or radius and the distance from the NMR tool to the wellbore wall to enable determination of the extent to which the region of investigation is within the borehole and/or within the formation.
As another example of a possible anomaly, the drilling tool can be off-axis with the borehole and additionally can be lying against one side of the borehole, permitting a portion of the sensitive region to full within the borehole cavity. In another example, the drill hole might have an elliptical cross-section rather than a circular one. In yet a third possibility, there can be a significant amount of washout, where certain segments of the borehole wall have separated and fallen away, leaving a cavity to one side of the borehole. Drilling muds typically have densities and structural nuclear values different from that of the surrounding rock formation. Contamination of wellbore signals in NMR by mud signals spoils all critical petrophysical estimates including porosity, permneability, and T2 distribution.
Pad or skid NMR tools have a depth of investigation of about 0.5xe2x80x3 to 1xe2x80x3. When the sensitive region lies so close to the surface of the borehole, as is the case in NMR logging, the rugosity of the surface becomes important. With a high rugosity, there is ample opportunity for mud to enter into the NMR sensitive region and permitting anomalous signals to contribute to the receiver formation signal. Borehole rugosity enables the mud volume to distort and infringe upon the measurements within the sensitive volume. Precise knowledge of this borehole rugosity can alert the practitioner that the data needs to be flagged.
Although corrections for invasive mud signals have been made in other logging methods, there is no similar method designed for use in NMR well logging. U.S. Pat. No. 3,321,625 (Wahl et al.) corrects for the effects of mud in gamma-gamma logging, using the knowledge of the mud density. U.S. Pat. No. 4,423,323 (Ellis et al.) addresses the problem in neutron logging. Thus there is a need for a method and apparatus for precisely determining borehole rugosity factors, comprising borehole caliper, stand off distance from a measuring tool to the bore hole wall and acoustic mud velocity and has been proposed in a separate application.
Physical limitations make it difficult to build side looking NMR tool that traverses a borehole and reads deeply into an adjacent formation. Typical NMR tools used in wireline logging operations read only 0.25 to 3 inches into the formation. However, typical pad or skid NMR tools, read only 0.5 to 1 inches into the formation. Thus, when deviations in the borehole wall extend partially or entirely into the sensitive volume of the NMR measurement, the NMR data are influenced by that portion of the NMR signal originating from the borehole. Thus, the NMR data collected with such tools can be corrupted by NMR signals originating from the borehole mud.
Gamma-Gamma density tools also are relatively shallow reading and their readings can also be adversely affected by even small-scale vertical rugosity, in which the skid is not capable of maintaining well bore contact. It is well known that such problems can be identified from density correction curves. No similar means of identifying questionable NMR data exists for data from the MRIL or CMR tools. Thus, there is a need to make a standoff measurement simultaneously with NMR tools. Typical standoff measurements have been made with a mechanical finger. Alternatively, an acoustic standoff measurement can be made. The standoff data can then be used to quality control the NMR measurements and flag data when the borehole rugosity (e.g. the depth of washout) exceeds the measurement sensitive volume. The mechanical standoff measurements are limited in accuracy, the amount of data points gathered and resolution.
It is preferable to not only identify data that is corrupted by NMR signal coming from the borehole, but to also correct such data. Data correction is discussed in U.S. patent application Ser. No. 09/896,463. To correct the data the standoff data is used to determine what percent of the data was corrupted by washout, which can be easily determined from the ratio of readings that exceed the depth of the sensitive volume to those that do not. The present invention then subtracts from the echo data, an echo train with the characteristic relaxation time of the borehole mud scaled to account for the amount of signal that originated from the borehole. This way both the NMR porosity and the properties derived from the echo train data will be substantially free of borehole contamination.
Acoustic standoff data requires precise acoustic velocity data to convert travel time to a precise distance. The velocity determination must be precise in order to achieve the desired accuracy of 0.1 inch in borehole diameter and 0.05 inch in standoff. Empirical correlation is available which accounts for pressure and temperature effects on the mud velocity. However, it is difficult to precisely know the makeup of the mud in the borehole at every depth and hence critical factors (e.g. water, oil and gas concentration, cuttings load, etc.) may not be known well enough to predict the mud velocity at every depth. Further, the addition of gas or suspended particles, cuttings, will also change the mud velocity. This renders measurements of standoff and/or borehole caliper based on acoustic time of flight measurements imprecise. Thus, there is a need for an accurate method and apparatus for measuring standoff and borehole caliper that does not suffer from variance due to variations in acoustic mud velocity.
The present invention addresses the shortcomings of the related art discussed above. The present invention provides a means for calibrating borehole standoff and caliper measurements. The present invention provides a fixed target that intersects only a portion of the acoustic beam and using the reflected signal from the fixed target to determine the mud velocity. It is not necessary to compute the mud velocity, as it suffices to divide the 2-way transit time to the borehole wall by the 2-way transit time to the fixed target to obtain the distance to the borehole wall. Of course, it is necessary to subtract the distance from the transducer to the reflector from the total signal path to obtain the actual standoff. The present invention provides a 45xc2x0 reflector to enable construction of a thin ultrasonic standoff or caliper measurement device. The present invention provides a combination of a 45xc2x0 reflector to reflect the signal to the formation and a 90xc2x0 reflector to measure mud velocity. In an alternative embodiment, the present invention provides a shaped (parabolic) 45xc2x0 reflector to focus the acoustic beam onto the borehole wall.
In one embodiment the present invention, the present invention determines the distance from the tool to the borehole wall, uses this distance to determine borehole invasion into the NMR sensitive volume. In another embodiment of the present invention, the present invention determines the distance from the tool to the borehole wall and corrects NMR data for the effect of the ROI invasion or overlap by borehole.
The present invention provides an acoustic standoff measurement wherein the two-way travel time is converted to distance. This conversion utilizes the precise measurement of mud velocity provided by the structure of the present invention. Mud velocity depends on the mud composition and pressure and temperature. The present invention provides a direct measurement of mud velocity and thus avoids the necessity of providing and using a second transducer for velocity measurements. In one embodiment of the present invention, the standoff transducer is recessed 0.5 to 1 inch inside the mandrel. Because the transducer is recessed, the minimum travel time will occur when the tool is flush against the borehole wall and the mud velocity can then be determined by dividing the minimum travel time by the distance the transducer is recessed. This enables conversion of the two-way travel-time data into standoff and to provide quality control and to correct NMR data for mud signal corruption. The present invention also measures signal amplitude and attenuation of the transmitted signal based on the amplitude of a return pulse from a fixed reflector and adjusts the transmitter pulse amplitude and/or receiver gain based on the amplitude from the fixed reflector to provide automatic gain control based on mud attenuation.